perhaps PJ is still prone to this sort of error but it is possible that this time he needs a younger guy because of the workload. They will try to make a dwarf fight the way we dream a dwarf should fight and that means more agile movements than what was achieved with Gimly, whose main fighting scene was CGI. It would take a lot of suspension of disbelief to watch a dwarf with restrained movements fight a spider and survive.

I think the young age of the actors means PJ will use CGI sparingly for his main dwarves. But the trade-off, as you pointed out, might be a less seasoned and kingly Thorin, even comparable (in age) to Gimly, when the difference between Thorin and LOTR-time Gimli is in fact like the difference between a man of 75 and a man of 50 years approximately.